A time-honoured children's game in Ankh-Morpork, a gutter sport that appears to have died out by the time of Thud. It involved placing informal bets on the speed of objects floating in water as they drift to an agreed finishing line.
It used to be called "Turd Races" until an attempt was made to take the name upmarket...
Clearly a reference to the game invented, or at least named, on Roundworld as "Poohsticks" by AA Milne in the Winnie the Pooh books. Each player drops a stick into the water on the upstream side of a bridge, and waits to see whose stick emerges from the other side of the bridge first.